The Bacillus subtilis tyrS gene is a member of a group of aminoacyl-tRNA synthetase and amino acid biosynthesis genes, designated the T box family, which are regulated by a unique transcription antitermination system. Expression of these genes is dependent on interaction of the leader region of the transcript with a specific uncharged tRNA. This interaction promotes formation of an antiterminator structure, preventing premature termination of transcription. At least 27 genes have been identified as members of this family in Gram-positive bacteria, including several pathogenic species. The T box mechanism has not yet been found in Gram negative bacteria. The tyrS gene is the best characterized member of this group. The specificity of the leader RNA-tRNA interaction is dependent on pairing of the anticodon of the tRNA with a single codon, the "specifier sequence," in the leader, and on pairing of the acceptor end of the tRNA with a portion of the antiterminator. These pairings are necessary for a successful interaction, but other features of the tRNA and the leader are likely to be important. One goal of this study is to further investigate this interaction, using both genetic and biochemical techniques. A number of conserved leader region elements are essential for antitermination. A second major goal is examination of the molecular role of these elements. While the tRNA-leader interaction is required for antitermination, it is not clear that this interaction is sufficient. A third major goal of this study is to identify any additional required factors by mutational analyses, and by reconstitution of the antitermination system in vitro and in a heterologous host. Overall these studies are designed to elucidate the molecular mechanism of T box gene antitermination.